Lofty valuations in global asset markets have caused a "wall of worry" for professional investors, but a lack of alternatives will mean equities will continue to climb in 2015, one strategist warned.

"Any correction will be met with a wall of buying, it's one of those moments," Peter Toogood, investment director at London-based independent fund manager City Financial Investment, told CNBC Monday.

"Take the U.S., there's nowhere to hide...the market in the U.S. is broadly insane."

He urged investors to "be nervous," but added that this nervousness and weak fundamentals weren't enough to stop the global rally in stocks and global fixed income. He predicted that the former would rally throughout the rest of the year.

"If you're about speculation, party on down," he said.

Record ratios

Getty Images

Traders work the floor of the New York Stock Exchange.

The comments come after Wells Capital Management highlighted at the start of the year that the median New York Stock Exchange (NYSE) stock was at a post-war record high in terms of its price-to earnings ratio, which is an important metric used by stock analysts to gauge a company's valuation. A high number can indicate that a company is more likely to be overvalued.

Market "froth" has been an ongoing concern for investors over the last few years. A bull run for equities is now in its sixth year on the back of extra liquidity provided by several central banks, including the U.S. Federal Reserve. The main concern for many economists is that quantitative easing (QE) may have artificially pushed stocks higher during a period where earnings and data fundamentals were relatively weak.

"To be honest, I am starting to get nervous because if you think about it, the earnings are still being downgraded," Alan Miller, chief investment officer at investment management firm SCM Direct.com, said in reference to both European and U.S. stock markets.

"Going forward it's hard to think markets can go up more than the earnings growth. So I think the rate of growth is definitely going to slow down and I think you have to be naturally more cautious," he told CNBC Monday.

Rise in buybacks

Many market participants also point to the rise in share buybacks as a key driver behind stocks' moves higher. Buybacks happen when firms purchase their own shares on the stock exchange, reducing the portion of stock in the hands of investors. They offer a way for companies to return cash to shareholders - along with dividends - and usually coincide with a company's stock moving higher as shares get scarcer.

These buybacks usually come at the expense of more capital investment by cash-heavy firms.

New data over the weekend from S&P Dow Jones Indices showed that shareholder returns reached more than $903 billion in the U.S. in 2014, according to the Financial Times. The same research also predicted that shareholders in the biggest U.S. companies could stand to receive a record $1 trillion in cash this year, according to the newspaper.

She told CNBC that companies were just "doing passive moves with capital," which would not keep them in a strong position over the coming years, and she urged more expenditure, development and investment in new markets.